These esters are destined for use as substitute of gas oil and must be of high purity, generally at least 97% esters content. On the other hand, after purification, it is desirable that the yield by weight of raw esters, in proportion to the oil and irrespective of the alcohol content in the ester phase, be at least equal to 95% and preferably from 97 to 102%.
Transesterification or alcoholysis of triglycerides or oils of vegetable or animal origin in the presence of an alcohol or acid or basic catalyst advantageously leads to the formation of fatty acid esters of alcohol and glycerol. Reference can be made in this respect to U.S. Pat. No. 4,164,506 and European Pat. No. 127,104.
The U.S. Pat. No. 2,383,601 recommends the use of absolute alcohol (methanol) in the presence of acid oils.
With absolute ethyl alcohol and acid oils (presence of fatty acids) or conversely with a neutral oil and hydrated alcohol, the transesterification by acid catalysis results in only a partial conversion rate. A certain amount of fatty acid is formed or remains which has to be neutralized in order to prevent any possible corrosion when the ester is used as a motor fuel. The neutralization results in the removal of said acids as salts but significantly reduces the yield to esters inasmuch as the latter are driven along with the sodium or potassium salts in an aqueous glycerol phase or even in the solid formed by the fatty acid salts.
The use of ethanol as a reactant therefor implies, according to several papers, that it is in anhydrous state and that the oils are neutral (J. GRAILLE in "Biomasse actualite" p. 35 June 1984).
In basic catalysis where generally the transesterification reaction is quick, neutral or weakly acid oils are used as starting materials since a too high proportion of fatty acids (&gt;0.5%) would inhibit the reaction by consuming catalyst to form soaps. According to FREEDMAN et al in JAOCS 61 No. 10 p. 1638, the catalyst mixture components, alcohol and oil, must be anhydrous since otherwise the yield to ester decreases, the reaction not being complete.
When using absolute alcohol and a neutral oil the problems are merely displaced. Industrial requirements for the drying of ethyl alcohol include sophisticaed equipment such as a plate column when the drying is achieved by an azeotropic carrying effect. When the alcohol is dried by means of molecular sieves, it is necessary, for example, on the basis of stoichiometrical ratio of 2, to use an amount of sieves of about 150 kg/t with an alcohol containing 7% of water. Moreover the oils must be subjected to an alkaline or physical refining which may be very costly in a small plant. Finally, with absolute alcohol, glycerol as formed does not settle spontaneously and the alcohol excess must be evaporated or water added, nevertheless not obtaining a complete conversion, the latter being generally obtained only with very high alcohol amounts. Finally, it should be very interesting to use hydrated ethyl alcohol in the manufacture of fatty acid esters.
As a matter of fact, the use of such an alcohol makes possible to consider the manufacture of gas oil substitute in a country or on a site where the industrial infrastructure is weak. For this purpose it suffices to manufacture an alcohol from fermentation products and to use a press to manufacture oil. An advantage of the use of hydrated alcohol is its low manufacturing cost.
Now, it has been surprisingly found that it was possible not only to use hydrated alcohol with success but that the use of said alcohol results in a simplification of the process for manufacturing the ester.
The process is simplified since, in the presence of hydrated alcohol, glycerol is easily settled and the ester phase is purer and the glycerol content lower even though the alcohol contains more water.